Greetings from the wet and windy Pacific Northwest -- the land of towering trees and spawning salmon.

Our dry summer is a distant memory, with only occasional glimpses of blue in our sky for the last few weeks. These days, the sun is something that we fantasize about rather than see. Perhaps the copious winter rainfall and our fondness for fish are responsible for our interest in green stormwater infrastructure. Green stormwater infrastructure relies on soils and plants to absorb the rain so that excess can enter the ground instead of streams and sewers. It is an option that many cities are turning to as a way to reduce the potential for combined sewer overflows without relying solely on engineered infrastructure.

There are many types of green infrastructure, ranging from rain gardens to swales in parking lots. In each case, the goal is to increase the infiltration rate of the soils and filter contaminants from the stormwater. A majority of these systems rely on engineered soils to accomplish this. A typical bioretention soil mixture consists of compost and sand, but a wide range of mixtures and specifications are out there, most based on best guesses rather than best science. In the case of Washington State, that status currently means that biosolids-based soil products are banned for use in these systems, as they are thought to be too rich in both metals and nutrients. This library gives you a selection of research on these systems, all from Northwest scientists and organizations.

We start with the salmon. Jenifer McIntyre has done some terrific research on the impacts of stormwater on salmon, first at NOAA and currently at Washington State University. This paper focuses on zebrafish instead of salmon, but it shows that passing stormwater through a bioretention soil mixture is sufficient to eliminate toxicity to the fish. These results have been replicated for salmon and are also available in a quick, easy to watch video produced by The Nature Conservancy: http://www.washingtonnature.org/cities/solvingstormwater/. Her research has also been replicated by other scientists outside of the Pacific Northwest (Anderson et al., 2016). The Anderson study (just ask, if you are interested) used stormwater collected from three bioswales in California that were made of topsoil, sand and compost over coarse aggregate. The mixture tested by McIntyre et al consisted of 60% sand, 15% compost, 15% shredded bark and 10% water treatment residuals (on a volume basis). These mixes both saved the fish. So, we know that these systems work. We don’t know exactly what in the stormwater was killing the fish or what parts of the soil mixture were critical for saving them.

The second article, here a literature review from the Seattle office of The Nature Conservancy, details how bioretention systems and other types of green infrastructure help save people. The publication details how exposure to nature in cities benefits public health. It is beautifully done and well referenced. It presents an optimistic view of what cities can be, with green stormwater infrastructure as one aspect of integrating nature into urban areas.

From here we go to the science of the swale. How do these systems work and what are the critical components of them?

The third article, from Washington State Universit, tested two types of compost added to the systems at the WA DOE recommended 40% by volume rate and measured leachate quality over 7 storm events. The stormwater here was collected from a roof at the WSU facility in Puyallup and was very clean. The authors found that the two composts were a source of N, P, and Cu for the first few events, with leachate concentrations of these elements decreasing over time. The authors used a speciation program that determined the form of Cu in the leachate. Due to high concentrations of dissolved organic matter, the majority of the Cu was predicted to be present in complexed form. Much of the concern in WA State on potential for fish mortality has been centered on total Cu concentrations in the leachate, with a limit set at 14 ppb for industrial discharge. In the McIntyre study, concentrations of Cu in leachate were higher than the benchmark, indicating that speciation may be key to fish survival.

The 4th article in the study by Palmer et al. includes Curtis Hinman as a co-author. Hinman, formerly of WSU, has been a key player in the bioretention field here in Washington State. His emphasis has been on identifying the best components for stormwater systems to get to net zero of just about everything in the leachate. This has often led to the use of imported materials such as coconut coir and specialized iron fibers. Our work with him has also shown that it often limits plant growth as well. Here he is looking at the addition of Al based WTRs (10%) and a saturated zone to enhance removal of P and N. The rest of the mixture consisted of 15% compost and 15% shredded bark. The addition of plants was also tested. Results showed that the WTR enhanced P removal and that the saturated zone increased nitrate removal. Plants were not shown to have a significant impact, but that was attributed to their small size and limited time to establish. The researchers also noted a decrease in nutrient export over time.

The last study in this month’s library was done by Julia Jay, an MS student with me at UW. It was funded by NW Biosolids, DC Water, and King County. Here we looked at a wide range of bioretention mixtures, including biosolids-based mixes from DC Water, biosolids compost from King County and DC Water, and yard/food compost (the current default material used in Western Washington with a focus on P movement). Composts were added +/- Fe based WTRs at different rates of addition. Sand alone was used as a control. Our goal was to develop predictive tools that could be used to evaluate mixtures based on their chemical properties rather than ingredients. The highest leaching occurred in the biosolids compost from King County. This was markedly reduced with addition of the WTRs. The lowest leaching was seen in the biosolids from DC Water. These materials have high concentrations of Fe, as Fe is added to remove P from the wastewater effluent. Previous work from our group had shown that the PSI (phosphorus saturation index), a ratio of oxalate extractable P to Fe + Al, was a good predictor of the potential to leach both P and Cu. Here we found that the PSR (phosphorus saturation ratio) based on the Mehlich III (a routine test for plant available nutrients) was much better across the wide range of mixtures we tested. Our group is currently working to take this one step further to figure out the best blends using different WTRs and composts and comparing these blends with different tests of availability.

I hope that this is helpful and I send my wishes to you from the soggy, but green, Northwest for a happy holiday. See you when the library starts back in February.